Not saying that the woman is not attractive, that's not the issue at hand. The "perv switch" is the switch that somehow gets flipped and causes a normally rational human to make a sexual decision that basically destroys their lives. Clinton is a great example. Or that story a year or two ago of that landlord who installed a camera in his tenants showers. Primally, most (rather, all) men would love to sow their oats in women less attractive than the Petrino chick. The fact is, most men don't.
For some however, the perv switch goes off and it's all over.